Fixed instancer loop logic that was causing Consul to get spammed

[jkratz55]

Address issue #1214 - Instancer spamming Consul with requests

The instancer loop was never updating the lastIndex but passing it to getInstances which caused Instancer to spam Consul repeatedly and nearly killed our Consul agent pods and time instances were added or removed from Consul for that service.

[peterbourgon]

Cool! Can you add a test which fails on the previous version and passes on this branch?

[jkratz55]

Hey @peterbourgon, I added a test case to the PR which fails on the old version and passes with the fix. Since the logic was in a loop that ran forever, the unit tests seem odd but seem to work. I'm just pumping out an error on a channel when the mock client detects the same index value was passed twice in a row and assumes every lookup Consul returns a new index since it isn't doing blocking queries in the mock.

[peterbourgon]

Thanks!